The misfolding and extracellular aggregation of destabilized, amyloidogenic proteins is inextricably linked to degenerative phenotypes in over 30 protein aggregation (i.e., amyloid) diseases including Alzheimer's disease, Creutzfeldt-Jakob disease and the systemic amyloidoses. Significant pharmacologic and genetic evidence confirms a causal relationship between protein aggregation and degeneration of post-mitotic tissues in these diseases. The importance of extracellular protein aggregation in amyloid disease pathology has stimulated significant experimental effort focused on defining the organismal and cellular pathways that regulate protein homeostasis (or proteostasis) in the extracellular environment. One such pathway is the Unfolded Protein Response (UPR) - the stress-responsive signaling pathways responsible for regulating proteostasis within the secretory pathway in response to endoplasmic reticulum (ER) stress. Previous results from our lab and others have shown that the UPR indirectly influences extracellular aggregation of destabilized, amyloidogenic proteins by reducing their secretion from mammalian cells, thus decreasing extracellular protein levels available for concentration-dependent aggregation. Here, we hypothesize that UPR activation also directly regulates extracellular proteostasis through the increased expression and secretion of extracellular chaperones that prevent the proteotoxic aggregation of destabilized, aggregation-prone proteins. We have identified the ER-targeted HSP40 co-chaperone ERdj3 as a UPR regulated, secreted chaperone that promotes extracellular proteostasis in response to ER stress. We show that ERdj3 attenuates the aggregation and proteotoxicity of disease-associated, aggregation-prone secreted proteins including A40 and toxic prion protein (TPrP). Additionally, we show that ERdj3 is co-secreted in a complex with destabilized, aggregation-prone proteins under conditions where ER proteostasis pathways are overwhelmed, providing a mechanism to preemptively protect the extracellular environment from proteotoxic extracellular protein aggregation. In this application, we expand on these findings using biophysical, biochemical and cell biological approaches to define the molecular mechanisms by which UPR-dependent ERdj3 secretion protects the extracellular environment against proteotoxic protein conformations. Through these efforts, we will identify specific aspects of UPR-regulated ERdj3 secretion directly involved in preventing the proteotoxic aggregation of destabilized secreted proteins associated with amyloid disease pathology. These results will demonstrate that altered UPR signaling, such as those that occur during normal aging or in response to amyloid-disease associated genetic mutations, can facilitate aging-dependent extracellular protein aggregation involved in amyloid disease pathogenesis. Furthermore, we will identify components of UPR signaling pathways that can be therapeutically targeted to promote extracellular proteostasis and prevent extracellular protein aggregation, revealing a new strategy to attenuate proteotoxicity of secreted proteins involved in the pathology of amyloid diseases.